Apple Scab Identification Guide

First Impression and Setting

Apple Scab announces itself as a change in the whole canopy before it shows as an individual lesion. Walk up to a crab apple in a hedgerow, a garden apple, or an orchard row in early summer after a wet spring, and the tree looks tired: leaves speckled with dull olive smudges, some yellowing, some curled, and a scatter of early leaf fall on the ground when the rest of the wood is still in full leaf.

Venturia inaequalis is an ascomycete confined to apples — domestic apple and the many ornamental and wild crab apples of the genus Malus. It is at its most conspicuous in regions with cool, wet springs, and its severity in any given year tracks how many prolonged wet periods coincided with young, expanding leaves.

Reading the Leaf Lesions

Pick a leaf and look at both faces, using a hand lens.

The earliest lesions appear on the undersides of the youngest leaves, and on the sepals and flower stalks of opening buds. They begin as small, pale olive-green patches with soft, feathery, indistinct margins that look as though something has been smudged onto the surface rather than grown from within.

As they expand to 5 to 15 mm they darken through olive-brown to nearly black, and their edges become sharply defined. The lesion surface is the key texture: under a lens it is not a flat stain but a low velvety pile, made of a dense turf of short erect spore-bearing stalks. Scrape it gently with a fingernail or a blade and a sooty film lifts off — a stain would not.

Heavily infected leaves pucker, curl, twist and distort, because the infected tissue stops expanding while the unaffected tissue around it continues. Later in the season the leaf yellows around the lesion while the lesion itself and a narrow ring of tissue around it stay green, producing a green-island effect that is very characteristic. Badly affected leaves drop early, and a tree can be substantially defoliated by late summer.

Reading the Fruit Lesions

On developing fruit the lesions start as the same olive velvet spots and then change character. They turn dark brown to black, become sharply circular, and the tissue beneath them goes corky, raised and rough. Because the scab tissue is rigid and the surrounding fruit continues to swell, the lesions crack, producing star-shaped fissures and sometimes deep splits.

Two other forms are worth recognising. Late-season infections produce pinpoint scab, a scatter of tiny black specks a millimetre or less across that only become obvious after the fruit has been in storage. Very early infections at the calyx end can distort the whole fruit into a lopsided shape.

Reading Twigs and Buds

On some crab apples, and less often on domestic apple, the fungus produces blistered, pustular lesions on the current season's shoots. The bark rises into small rough blisters that later rupture, and these lesions can carry the fungus through winter on the tree itself.

The Overwintering Stage

This is the section that seals the identification, and it is done in late winter or early spring on last year's fallen leaves.

Gather leaves that have overwintered on the ground beneath an affected tree. Hold them to the light and examine the old lesion areas with a lens: embedded in the leaf tissue are minute black flask-shaped fruiting bodies, the pseudothecia, roughly 100 to 150 micrometres across, appearing as tiny raised black dots. Around the mouth of each is a fringe of short stiff dark bristles, and that bristled ostiole is a genuinely useful character for the genus.

Reading the Spores

There is no spore print to take from a plant pathogen, but the equivalent procedures are quick.

For the summer stage, press a strip of clear adhesive tape sticky-side down onto a velvety lesion, lift it, and mount it on a slide in a drop of water. Under the microscope you should see olive-brown conidia, pear-shaped to club-shaped with a tapering apex, borne singly on short stalks that show stepped scars where successive spores were shed. They are roughly 12 to 22 by 6 to 9 micrometres, one-celled at first and becoming two-celled.

For the overwintering stage, float infected leaves on water in spring and examine crushed pseudothecia. Inside are cylindrical to club-shaped asci, each holding eight ascospores. Those ascospores are olive-brown and divided by a single cross wall into two unequal cells — one distinctly larger than the other. That inequality is the origin of the species epithet and is the single most satisfying confirmation available.

Season and Life Cycle

The rhythm is worth knowing because it explains where to look and when. Pseudothecia mature in fallen leaves over winter. In spring, from bud burst through to a few weeks after petal fall, rain triggers forcible discharge of ascospores into the air, and these are the primary infections; they need a sustained period of leaf wetness at moderate temperatures to establish. Lesions appear one to three weeks later, and from then on each lesion produces conidia that are splashed to new leaves and fruit, giving repeated secondary cycles all summer whenever the canopy stays wet. At leaf fall the fungus switches back into the sexual stage in the dead tissue.

Separating It From Look-Alikes

Sooty blotch and flyspeck, a complex of superficial fungi on fruit skin, produce either diffuse smudgy olive-grey blotches with indefinite edges or tight groups of small shiny round black dots. Both sit on the surface and can be rubbed or washed off, whereas scab lesions are embedded, corky and cracked.

Cedar-apple rust and its relatives make bright yellow to orange leaf spots with tiny black dots on the upper surface and tubular or hair-like fringed structures beneath. The colour alone rules out scab.

Frogeye leaf spot produces round lesions with purple margins and tan centres that often show concentric zoning, with a distinctly crisp outline and no velvety pile.

Powdery mildew coats young shoots, leaves and buds in a white to grey powder and distorts the growing tips; it is white, not olive.

Marssonina blotch creates dark spots with radiating fibrous margins on the upper leaf surface and drives heavy yellowing and defoliation, but lacks corky fruit scabs.

Pear scab, caused by a close relative, produces near-identical lesions but only on pear.

Physical russeting, hail scars and rub marks on fruit can look scabby but are smooth-edged, distributed by exposure rather than around the calyx, and never begin as velvety olive spots.

Field Checklist

Malus host; olive velvety leaf lesions with feathery margins hardening into sharply defined black patches; leaf distortion, green islands and early leaf drop; corky, cracked, sharply circular fruit lesions; bristle-mouthed black pseudothecia in overwintered fallen leaves; olive-brown two-celled ascospores with unequal cells.